def divisor(n):
    ass = []
    for i in range(1,int(n**0.5)+1):
        if n%i == 0:
            ass.append(i)
            if i**2 == n:
                continue
            ass.append(n//i)
    return ass

def main():
    A, B = map(int, input().split())
    C_cand = divisor(A+B)
    sort_C = sorted(C_cand)
    for C in sort_C:
        if C in [A,B]:
            continue
        if (B + C) % A == 0 and (A + C) % B == 0 and (A + B) % C == 0:
            print(C)
            exit()

    print(-1)



if __name__ == "__main__":
    main()